1. Field of the Invention
This invention relates to coated paper for use in so-called "clean-to-handle" pressure-sensitive copying systems.
2. Description of the Prior Art
One known clean-to-handle pressure-sensitive copying system comprises an upper sheet, known as a CB sheet, which is coated on its lower surface with pressure-rupturable microcapsules containing a solution of a colour former material, and a lower sheet, known as a CF sheet, which is coated on its upper surface with a colour reactant material, such as an acidic clay or a phenolic resin. For most applications, a number of intermediate sheets, known as CFB sheets, are also provided each of which is coated on its lower surface with microcapsules and on its upper surface with colour reactant material. The pressure exerted on the sheets by writing or typing ruptures the microcapsules, thereby releasing the colour former solution onto the reactant material on the next lower sheet and giving rise to a chemical reaction which develops the colour of the colour former. The microcapsules are usually applied in aqueous suspension.
The above described pressure-sensitive copying system will hereinafter be called a "pressure-sensitive copying system of the kind referred to".
CF sheets in which the colour reactant material is an acidic clay have hitherto been manufactured by coating, e.g. blade coating, a suitable base paper with an aqueous coating mix, and for this purpose it is desirable for the base paper to have as smooth a surface as possible, since a smooth surface allows a low colour reactant material coat weight to be used, whilst still affording the desired CF properties. The desired smoothness is normally obtained by calendering the base paper prior to coating thereof, and such calendering also serves to make the surface of the base paper compact, which minimises migration of the subsequently applied coating into the base paper. The base paper is normally calendered again after coating in order to make the coating compact and give it a smooth surface. A smooth CF surface is desirable since when a CB or CFB sheet overlies the CF sheet there will be a tendency for any roughnesses of the CF surface to snag the microcapsules on the CB surface, which may lead to premature rupture of the microcapsules, or "smudging" as it is usually known.
Similarly, base paper for the manufacture of CB sheets has hitherto been calendered to provide a compact surface prior to application of an aqueous microcapsule dispersion thereto. The microcapsule dispersion may for example be applied by means of an air-knife coater. However, the smoothness resulting from calendering of the CB sheet base paper would lead to the microcapsules standing out from the surface of the base paper, in which position they would be very prone to accidental rupture, for example during handling operations. It has therefore become a normal practice to coat the microcapsules onto the surface together with a so-called stilt material, such as cellulose fibres or starch granules, the dimensions of which are such that the stilt material protrudes further from the base paper than the microcapsules. The stilt material therefore serves to protect the microcapsules against accidental rupture while still allowing rupture under typing or writing pressure.
The presence of stilt material adds to the expense of CB and CFB sheets, and may also lead to problems in coating the base paper with microcapsule suspension.